The regeneration of waste paper such as used newspapers and magazines has been performed for a long time. Recent shortage in pulp resources and a rise in the pulp price have further elevated the importance of the effective utilization of waste paper. Furthermore, the application of deinked pulp has been enlarged and advanced. On the other hand, recent waste paper differs from conventional ones in the printing techniques, printing systems and ingredients of printing inks. Furthermore, waste paper which was never recovered and regenerated is now employed. As a result, the regeneration of the waste paper becomes more and more difficult, from the viewpoint of deinking. In order to promote deinking, therefore, attempts have been made to improve deinking devices.
These attempts comprise, for example, providing a tower aiming at high concentration aging, using a despicer or a kneader for promoting the liberation of an ink through the application of a physical force or providing a high consistency defibering pulper.
Examples of chemicals, which have been employed for separating and removing inks and other impurities from waste paper for a long time, include alkaline agents (for example, caustic soda, sodium silicate, sodium carbonate, sodium phosphate), bleaching agents (for example, hydrogen peroxide, hyposulfite, hypochlorite), sequestering agents (for example, EDTA, DTPA) and deinking agents (for example, anionic surfactants such as alkylbenzenesulfonates, higher alcohols sulfuric acid esters, .alpha.-olefinsulfonates, dialkylsulfosuccinates; nonionic surfactants such as ethylene oxide adducts of higher alcohols, alkyl phenols and fatty acids, alkanolamides). One of these substances or a mixture thereof have been used. Although these deinking agents are excellent in foamability at the flotation treatment, they are poor in the ability of collecting inks. In the case of the washing system, furthermore, the detergencies of these deinking agents are limited. In addition, the high foamability causes a trouble of foaming at the drainage step. Therefore, the deinked pulp obtained by using these deinking agents has poor qualities. Even though the resulting deinked pulp has a high degree of whiteness, it looks dark and dull, which reduces the amount of the deinked pulp to be used, for example, in millboard, newspapers. Alternately, in order to reduce darkness and dullness of the resulting deinked pulp, it is required to add a bleaching agent thereto in an elevated amount.